The Fischer Tropsch (FT) process, which can be described as a heterogeneous surface catalysed polymerisation reaction, usually entails the hydrogenation of carbon oxide (typically either carbon dioxide, carbon monoxide, or a mixture thereof) in the presence of a catalyst based on Group VIII metals, such as iron, cobalt and ruthenium. Depending on the particular reaction conditions, the products formed from this reaction may be water, gaseous, liquid and waxy hydrocarbons which may be saturated or unsaturated. Oxygenates of the hydrocarbons such as alcohols, acids, ketones and aldehydes can also be formed. The carbon number distribution of the products follows the well-known Anderson-Schulz-Flory distribution.
Where the carbon oxide comprises carbon monoxide, the reaction can be represented by the following equation:nCO+2nH2→(CH2)n+nH2O
Such heterogeneous Fischer Tropsch processes are usually referred to as being either a high temperature Fischer Tropsch (HTFT) process or a low temperature Fischer Tropsch (LTFT) process.
The HTFT process is usually carried out at temperatures from 250° C. to 400° C. and the catalyst employed is usually a fused iron-based catalyst, but precipitated iron-based catalysts can also be employed. At such temperatures, both the reactants and the products are in a gas phase in the reaction zone and, with the catalyst being in the solid form, the process can be referred to as a two phase FT-reaction. The process is usually commercially carried out in a fluidised bed reactor and the products obtained are of relatively high olefinicity and shorter chain length (that is, products in the gasoline and diesel range) compared to LTFT processes employing an iron catalyst.
The LTFT process is usually carried out at temperatures from 180° C. to 310° C. and the catalyst employed is usually a Co-based catalyst, although a Fe-based catalyst can also be used. The conditions under which this process is carried out, results in at least some of the products being in a liquid phase in the reactor. With the reactants being in the gas phase, at least some of the products in the liquid phase and the catalyst being solid, this process can be referred to as a three-phase process. The process is usually commercially carried out in a fixed bed reactor or a slurry bed reactor and the products obtained are heavier hydrocarbons such as waxes. A fluidised bed reactor cannot be used in this process, as the liquid product causes adhesion of the solid catalyst particles to each other, which will affect the fluidization properties of the catalyst.
Because the HTFT and LTFT processes are different, the catalyst used in each of the processes is accordingly usually also different. The catalyst is generally optimised for a specific process and for the attainment of a specific range of products.
It is usually commercially desirous to be able to produce iron based catalysts for both the LTFT and HTFT reactions from a substantially pure source of iron oxide.
The preparation of such iron oxides is well known in the art, and some of such methods date back to those described in U.S. Pat. Nos. 1,327,061 and 1,368,748 according to which metallic iron is immersed in a solution of a soluble ferrous or ferric salt into which an oxidising agent such as air is introduced to precipitate the desired ferric salt.
A disadvantage associated with the aforesaid known processes described in said two US patents is that the metal dissolution, oxidation and hydrolysis all take place in the same vessel and at the same time. Dissolution rates, oxidation rates and hydrolysis rates are therefore difficult to control individually and can accordingly give rise to undesired forms of iron being formed.
Another prior known process, disclosed in U.S. Pat. No. 6,790,274, entails the dissolution of Fe(0) in an acidic medium in which the acid to iron ratio is less than 3:1, oxidation of the resultant Fe(II) to Fe(III), and the hydrolysis, and consequential precipitation of the Fe(III), all done in the same vessel.
It is accordingly extremely difficult in this process to do rapid oxidation of all of the iron as well as rapid precipitation of the formed Fe(III) since the rate determining step is the dissolution of Fe(0) through the in situ redox couple of Fe(0)+2Fe(III)→3Fe(II) whereby some of the formed Fe(III) is reduced to Fe(II) again. A disadvantage associated with this is that additional oxidising agent is required to convert the Fe(II) so formed back to Fe(III). As a result, some of the oxidising agent is accordingly indirectly used to oxidise Fe(0) to Fe(II).
A further disadvantage found with the aforesaid process of U.S. Pat. No. 6,790,274 is that it keeps on generating Fe(II) in solution which slows down the net rate of formation of Fe(III) in solution. A yet further disadvantage is that due to the aforesaid redox couple it is extremely difficult to perform hydrolysis and/or precipitation of exclusively Fe(III) from the solution.
Methods for the production of iron based Fischer Tropsch catalysts are also known in the art. In one such method, disclosed in U.S. Pat. No. 7,199,077, the process comprises the steps of preparing an aqueous solution of carboxylic acid and water and adding iron metal thereto, thereafter forcing an oxidising agent through the acidic solution to consume the iron and to form an iron oxide slurry. The slurry is then milled and promoters added thereto. The product is then spray dried and calcined to form the catalyst.
Because the dissolution, oxidation and hydrolysis of the metal in this process is carried out at the same time and in the same vessel, incomplete dissolution of the iron metal in the acidic solution prior to oxidation can occur. This means that promoters that can be involved in a redox couple with Fe(0) (for instance copper) may only be introduced to the process after the iron slurry is removed from the reaction vessel, because the promoters could otherwise interfere with the dissolution of the iron.
The Applicant has now found a method of producing an iron-based catalyst precursor and/or catalyst that reduces and/or eliminates at least some of the aforesaid disadvantages. Furthermore, it has also been found that such a catalyst, on activation, can successfully be used in a FT-process, particularly a LTFT process for the synthesis of hydrocarbons from syngas.